Lowering pH below the critical pH promotes caries progression by which mechanism?

• A. Promoting growth of beneficial bacteria
• B. Inhibiting growth of aciduric bacteria
• C. Inhibiting growth of beneficial bacteria and promoting aciduric bacteria
• D. Has no effect on microbial balance

Answer: (C)

Low pH creates an acidic biofilm environment that shifts the microbial balance toward aciduric and acidogenic bacteria. When the environment becomes more acidic, many non-aciduric or beneficial bacteria are suppressed, while acid-tolerant organisms like Streptococcus mutans and lactobacilli thrive and continue to metabolize sugars into acids. This ongoing acid production lowers the pH further, promoting demineralization of tooth structure and accelerating caries progression. So the mechanism is that lowering pH inhibits beneficial bacteria and promotes aciduric bacteria. The other options don’t fit because they would either reduce acid production, fail to shift the balance, or ignore the ecological response to a low pH.